To be fair, you can't blame the Scarlet Crusaders from not distinguishing between Forsaken and Scourge when a significant fraction of the Forsaken faction was exactly as evil and destructive as the Scourge.
Yes, many members of the Forsaken aren't evil, but when you have no way of knowing which is which (because they don't very well have uniforms distinguishing the bad from the good), and they all get violently angry when you kill members of their faction (because the benevolent Forsaken don't really know about the malevolent ones), it's no surprise that the Scarlet Crusaders just said "hell with it, kill 'em all".
Fanaticism and demonic infiltration/corruption didn't help though, to be sure.